There has been much work devoted to the alignment of lasers (or photodetectors) with optical fibers to provide for the maximum amount of coupling and subsequent transmission of light along an optical fiber. Most products are based on a single laser aligned to a single optical fiber—an example is the SFP optical transceiver, with a well aligned transmitter module and a well aligned receiver module; each with a single optical fiber coupling in an LC connector housing.
However, due to ever growing demands for bandwidth in smaller overall volumes of space (known as a higher bandwidth-density) there has been an increasing trend towards parallel optical modules where multiple lasers are aligned to multiple optical fibers in a single module—an example of this is the SNAP-12 parallel optical transceiver with 12 optical channels in an area roughly 1-cm×1-cm. Both single and multiple optical channel modules require well positioned and well toleranced sub-components and holders—such as described by Gallup, et al., U.S. Pat. No. 6,955,480 granted Oct. 18, 2005 and Kanazawa, et al., U.S. Pat. No. 6,179,483 granted Jan. 30, 2001. Furthermore, they are usually aligned while actively monitoring the optical power levels, to or from the optical fiber, to ensure the highest possible coupling (lowest insertion loss)—this is especially true for modules with long link distances (>1-km) that are based on single mode optical fiber, but also is needed for modules with shorter link distances (<300-m) that are based on multimode optical fiber.
The principle behind the alignment of the parallel optical module has been to use several levels of aligned module or components starting with photo-lithographically defined patterned arrays of lasers (or photodetectors). These arrays can be made with extremely high resolution, typically better than 1-micron tolerance for 12 lasers on a 250-micron pitch. These VCSEL arrays of lasers are produced as single chips roughly 3-mm×0.3-mm×0.15-mm with 12 lasers in a row. The alignment between the laser array and the optical fibers requires a more complicated set of parallel holders and relay optics—such as a patterned microlens array and an MT-style optical ferrule—but requires the same alignment methodology as the single laser module—just done on both ends of the array at the same time. The difficulty here is that alignment is time consuming, requires additional components (like relay microlenses with dowel pin alignment post), requires the lasers to be powered-on for active alignment, and requires precision pick-and-place techniques to place the VCSEL chips.
A further complication to this alignment strategy for parallel optical modules, has been an increasing demand to include both out-going and in-coming optical signals within the same single, small form-factor module. This requires that both the laser arrays (VCSELs) and photodetector arrays (PDs) be placed along side each other where a single high-density optical connector (such as the MT ferrule) is used to maintain the bandwidth-density advantage of the module. A few examples of modules which have both out-going and in-coming optical data signals are the POP-4 transceiver and the QSFP transceiver. This type of alignment requires three independent parts to be aligned with respect to each other where typically the VCSEL and PD arrays are aligned together, such that they are pitched on 250-micron centers and in co-linear position of their active areas (and not necessarily the sides of the chips). This further compounds the earlier active alignment issues as describe above and increases the alignment time, the set-up time and the accuracy of the equipment required.